Comment 4 for bug 1796661

The issue is because firefox does a rename from the /tmp folder to the encrypted home folder, right now fscrypt returns EPERM instead of EXDEV that would be normal when the operation can not complete because of crossing a mount point or in this case unencrypted to an encrypted folder.

The upstream firefox bug is here:
https://bugzilla.mozilla.org/show_bug.cgi?id=1521041

But one could argue that the kernel interface is broken and it should return EXDEV as tools like mv also breaks, there are some patches in the works but I don't know how far they are, maybe ubuntu could carry them until they go upstream:

https://patchwork.kernel.org/patch/9943001/